Process for producing a valve seat

ABSTRACT

The invention relates to a process for producing a valve seat for a cylinder head of an internal combustion engine, in which an additional material is fused to the cylinder head, through introduction of energy, at the location at which the valve seat is to be formed, wherein the additional material used is a copper alloy which, in addition to copper, comprises the following elements:  
                                           iron less than    5% by weight,         manganese   10% by weight-20% by weight,         cobalt    5% by weight-10% by weight,         molybdenum    5% by weight, 9% by weight, 5% by             weight,         boron    1% by weight-3% by weight,         chromium less than    3% by weight.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to a process for producing a valve seat for acylinder head in accordance with the preamble of claim 1.

2. Related Art of the Invention

EP 1 120 472 A1 describes a process for producing a valve seat for acylinder head. In this process, an additional material is applied arounda valve opening through a nozzle, with energy additionally beingintroduced along the valve opening by a separate laser element and thepulverulent additional material and the material of the cylinder headbeing partially melted at this location so that they are fused together.The additional material then forms an alloy, therefore, with thematerial of the cylinder head, and consequently forms a localreinforcement in this region, which in turn forms the valve seat.

The material which is used in this process consists of a copper alloywhich contains 6-9% by weight of nickel, 1-5% by weight of silicon, 1-5%by weight of molybdenum and also tungsten, tantalum and niobium.

The valve seat at the cylinder head of internal combustion enginesserves the purpose of sealing off the combustion chamber with respect tothe environment. The valve presses onto the valve seat, producing a highlevel of wear at the valve seat, which means that it is necessary to usehighly wear-resistant materials in this region. At the same time, themaximum possible quantity of heat should be dissipated from the regionof the sealing surface and the combustion chamber to the water jacket.

The process and alloy described in EP 1 120 472 A2 satisfy these demandsby using a copper alloy so that at least a good thermal conductivity isachieved, but the wear resistance at the valve seat is still inadequate.

SUMMARY OF THE INVENTION

The invention is based on the object of providing a process forproducing a valve seat, the valve seat having a high thermalconductivity and also having a significantly increased wear resistancecompared to the prior art.

The solution to the object consists in a process for producing a valveseat in accordance with the features of patent claim 1.

In the process according to the invention for producing a valve seat fora cylinder head of an internal combustion engine, an additional materialis applied to the cylinder head and fused to the material of thecylinder head at the location at which the valve seat is to be formed,as a result of energy being introduced. According to the invention, theadditional material used in this process is a copper alloy, the alloycomprising, in addition to copper, the following elements: iron lessthan  5% by weight, manganese 10% by weight-20% by weight, cobalt  5% byweight-10% by weight, molybdenum less than  5% by weight, nickel lessthan  9% by weight, silicon less than  5% by weight, boron  1% byweight-3% by weight, chromium less than  3% by weight, and inevitableimpurities.

With the alloy composition according to the invention, the principalelement copper substantially has the effect of producing a good thermalconductivity and bonding to the base material. The alloying element ironhas the effect of increasing strength, but a maximum concentration(solubility limit) of 5% for iron in copper should not be exceeded. Thealloying element manganese is particularly good at increasing thestrength of the alloy. If manganese forms from 10-20% of the alloy,superstructures which have a positive influence on the hot strength andmoreover lead to a higher wear resistance are formed.

The alloying element cobalt likewise produces a higher resistance towear and, moreover, contributes to grain refining. Molybdenum as analloying element acts as a solid lubricant. The lubricating action isensured by the molybdenum compounds MoS₂ and MoO₃. At the same time, themolybdenum interacts with the silicon, leading to the formation ofadvantageous silicides. The alloying element nickel also formssilicides, and moreover leads to the formation of a solid solution withthe copper, based on the nickel being completely soluble in the coppermatrix.

As has already been mentioned, the alloying element silicon formscompounds with the alloying elements nickel and molybdenum, producingwhat are known as silicides. The silicon improves the wettability of theadditional material with the partially melted base material. Thealloying element boron also contributes to improving the wettability.

It should be noted that additional aluminum is supplied to the systemfrom the aluminum-silicon alloy of the base material as a result of thepartial melting of the latter.

The alloying element chromium is only slightly soluble in copper. Itforms hard materials, such as Cr₂C₃ and silicides, in the alloy. Thesecompounds make a contribution to the hardness of the valve seat.However, the chromium content must be stoichiometrically matched to thesilicon content.

In one advantageous embodiment of the invention, the additionalmaterial, as well as copper, includes the following elements: iron   2%by weight-4% by weight, manganese 11.5% by weight-14% by weight, cobalt  5% by weight-10% by weight, molybdenum   2% by weight-4% by weight,nickel   3% by weight-6% by weight, silicon   2% by weight-4% by weight,boron  1.5% by weight-2.5% by weight, chromium   1% by weight-2% byweight, and inevitable impurities.

It has emerged that a manganese content of between 11.5% by weight and14% by weight makes a particularly good contribution to increasing thestrength and to the wear resistance without having any adverse effect onthe action of the other alloying elements.

In a further embodiment of the invention, the additional material, aswell as copper, includes the following elements: iron  3% by weight,manganese 12% by weight, cobalt  5% by weight, molybdenum  3% by weight,nickel  3% by weight, silicon  2% by weight, boron  1% by weight,chromium  1% by weight, and inevitable impurities.

The percentages indicated for the alloying elements are in each case tobe understood as being within the context of manufacturing accuracy.When producing the alloy, an inaccuracy of ±0.5% by weight per alloyingelement is in each case assumed.

In the context of the invention, it may be expedient for the additionalmaterial to contain up to 15% by weight of tin (Sn), which forms a CuSnmatrix with the copper and in this way increases the basic strength ofthe material. The addition of phosphorus (P) also contributes to theformation of CuFeP solid solutions and therefore to an increase in thehot strength.

In one embodiment of the invention, the energy is introduced by a laserbeam. The laser beam and the additional material are fed to the locationof action through a common unit comprising focusing optics and a coaxialnozzle. This ensures that the introduction of energy and the supply ofmaterial are always locally matched to one another.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are explained in more detail onthe basis of the following drawings, in which:

FIG. 1 shows an excerpt from a cylinder head in the region of the valveseat,

FIG. 2 shows an enlarged excerpt from the valve seat with valves,

FIG. 3 shows the application of the additional material and of the laserbeam through a coaxial nozzle,

FIG. 4 shows a cross section through a melt track at a cylinder headsurface.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows part of a cylinder head 1 of an internal combustion engine,which is not illustrated overall. The cylinder head 1 has an intake duct2, in a manner which is known per se, which in the present case could,of course, also be formed as an exhaust duct. The intake duct 2 isclosed and opened by a gas exchange valve 3, referred to below as valve3 for the sake of simplicity, so that air or a fuel/air mix can enter acombustion chamber 4 of the cylinder head 1 from the intake duct 2.

The cylinder head 1 is provided with a valve seat 5 against which thevalve 3 bears in its closed position, so as to disconnect the intakeduct 2 from the combustion chamber 4.

The valve seat 5 is arranged annularly around a valve opening 6 in thecylinder head.

To produce the valve seat 5, the laser light from an Nd:YAG laser ispassed through a glass fiber to focusing optics (not shown in moredetail here). The use of a glass fiber makes it easy to guide the laserlight to the processing location, with the result that the process costsand the systems outlay can be reduced.

The focusing optics of the laser beam are connected to a coaxial nozzle8, as illustrated in FIG. 3. The coaxial nozzle 8 comprises two outlets,an inner opening 12 and an outer opening 14. The laser beam 10 is guidedthrough the inner opening 12. The additional material 16 is guidedthrough the outer opening 14. This arrangement has the advantage thatthe energy source, the laser beam 10, and the additional material, whichforms the valve seat 5, are always guided onto the same location. Thismakes the processing operation direction-independent, so that additionaloutlay on equipment if the laser beam and additional material were toform a preferred direction can be saved.

The focusing optics shape the laser beam 10 in such a way that on thecylinder head 1 it has a focal point with a diameter of approximately2-5 mm. The quantity of energy which acts on the cylinder head 1 at thisfocal point is preferably metered in such a way that the cylinder headmaterial is partially melted at this location. The width of the focalpoint is designed in such a way that the entire width of the valve seatcan be formed by one track of the laser. In this case, the processparameters energy density, diameter of the focal point, advance rate ofthe laser or the coaxial nozzle around the valve opening 6 and thedelivery quantity of additional material have a combined effect. Theseparameters have to be set in such a manner that the desired melting canbe achieved. The coaxial nozzle 8 containing the laser beam 10 and theadditional material 16 is therefore moved along the valve opening 6 inthe shape of a circle. The advance is in this case between 300 mm perminute and 1000 mm per minute. In the process, the surface of thecylinder head 1 is partially melted in the region of the valve opening6. The additional material 16, which is supplied from the outer opening14 of the coaxial nozzle 8, is likewise partially melted by the laserenergy. The molten cylinder head material and additional material forman alloy. FIG. 4 shows a cross section through a melt track, with line20 marking the surface of the cylinder head 1.

The advance rate of the processing, on a production engineering scale,is between 300 mm per minute and 1000 mm per minute. The advance rate isin this case dependent on the geometry of the cylinder head 1 at thevalve opening 6.

The mixing depth of the cylinder head surface is defined in thefollowing way by the features of FIG. 4:

-   -   mixing depth=(cross section 24 beneath the cylinder head surface        20/overall cross section 26 of the melt track 18)×100%.

The molten material of the cylinder head and the molten additionalmaterial are combined with one another in a transition layer (mixedlayer). The thickness of the transition layer is usually less than 1000μm. The transition layer, which is not illustrated in FIG. 4, does notnecessarily have to coincide with the cylinder head surface 20; it maybe at a higher or lower level. Accordingly, only the additional materialforms the surface of the melt track 18 and therefore the surface of thevalve seat 5. The functional properties of the valve seat 5 aretherefore produced exclusively by the additional material 16. Thepartial melting of the cylinder head surface 20 serves substantially toensure metallurgical bonding (fixed, non-brittle bonding) between theadditional material and the cylinder head 1.

To avoid changes in the properties of the cylinder head material, inparticular in the case of very thin webs, as also occur in the region ofthe valve opening 6, excessively deep mixing is to be avoided. Accordingto the relationship given above, the mixing depth should be less than30%, preferably less than 20%.

1. A process for producing a valve seat for a cylinder head of aninternal combustion engine, in which an additional material is fused tothe cylinder head, through introduction of energy, at the location atwhich the valve seat is to be formed, wherein the additional materialused is a copper alloy which, in addition to copper, comprises thefollowing elements: iron less than  5% by weight, manganese 10% byweight-20% by weight, cobalt  5% by weight-10% by weight, molybdenumless than  5% by weight, nickel less than 9% by weight, silicon lessthan  5% by weight, boron  1% by weight-3% by weight, chromium less than 3% by weight, and inevitable impurities.


2. The process according to claim 1, wherein the additional material, aswell as copper, comprises the following elements: iron   2% by weight-4%by weight, manganese 11.5% by weight-14% by weight, cobalt   5% byweight-10% by weight, molybdenum   2% by weight-4% by weight, nickel  3% by weight-6% by weight, silicon   2% by weight-4% by weight, boron 1.5% by weight-2.5% by weight, chromium   1% by weight-2% by weight,and inevitable impurities.


3. The process according to claim 1, wherein the additional material, aswell as copper, comprises the following elements: iron  3% by weight,manganese 12% by weight, cobalt  5% by weight, molybdenum  3% by weight,nickel  3% by weight, silicon  2% by weight, boron  1% by weight,chromium  1% by weight, and inevitable impurities.


4. The process according to claim 1, wherein the additional materialcontains between 0% by weight and 15% by weight of tin.
 5. The processaccording to claim 1, wherein the additional material contains between0.05% by weight and 0.3% by weight of phosphorus.
 6. The processaccording to claim 1, wherein the energy is introduced by means of alaser beam, and the laser beam and, the additional material pass ontothe valve seat together through a coaxial nozzle.